Thermal Expansion Illustrated with Video Microscopy

This is the expansion of a 87mm aluminum tube as it is heated from approximately 20C to 100C. The calibration was inserted with the Proscope software.

Topics addressed

Description of Investigation

Thermal Expansion

This is a variation to the traditional linear thermal expansion demonstration. Normally, thermal expansion is measured via some sort of mechanical instrument, such as a dial indicator. In this variation, the expansion is made visible through the use of a video microscope. The video microscope is positioned at the free end of the tube being expanded and focused to allow you to see the relative motion of the tube over its support. In this investigation you won't be measuring the actual expansion, but you will be examining how and why the material expands as it does.

Study Guide:

Set up the apparatus as shown in these instructions. Instead of using the pointer, set up the video microscope on a tripod and aim the microscope at the free end of the tube. Make sure that you can see both the tube and the wood below it for a reference point. It is very helpful to place a millimeter scale beneath the free end of the tube to give a visual reference for the movement.

Ice water is first run through the tube to get it as cold as possible. Make sure that the mark has stopped moving before moving to the next step.

Pour hot water through the tube, carefully observing how the mark moves. Of particular interest is whether the mark moves at constant speed or if it accelerates and decelerates at the ends of its travel. Pour sufficient water to make sure that the mark has stopped moving.

Allow the tube to sit for a couple of minutes while observing the microscope image. Does the metal stay expanded, or does it start to contract? Does it return to its original position?

Now pour ice water through the tube and observe the microscope image. Does the mark return to its original position?

Questions:

When the hot water was poured through the tube, did the mark move at a constant speed or did it accelerate and decelerate? Why did it move as you observed?

When the cold water was poured through the tube, did the mark return to its original position? Was this your expected result?

What would happen if a colder material, such as alcohol cooled with dry ice, were passed through the tube?

Here is the basic setup. An aluminum tube is secured to the near end of the 2x4 with rubber bands. A funnel and hose is arranged to allow hot water to be poured through the tube. At the other end of the tube a short hose carries the water into a sink.

The Scalar Proscope with a 50x lens is positioned at the free end of the tube so that both the tube and the supporting 2x4 are visible. The ProScope is supported by a photographer's tripod.

Here is the microscope's view as shown on the computer display. A red line has been drawn on the tube and matched with the millimeter scale of a small ruler to make the movement more apparent. It is difficult to get both the tube and scale in focus simultaneously, but an acceptable image is possible..

Here is the computer display. This display can be projected onto a large display for the entire class to view.

A screenshot of the computer display. The image can also be shown in full screen mode for better visibility. Using the ProScope software, still images or Quicktime movies can be saved at any time. This image shows the setup without the millimeter ruler inserted under the tube.

References & Links:

Instructions to build the thermal expansion apparatus for less than $10 can be found here.

If you have money to burn, here's a very nice apparatus. It doesn't really work any better, though.